1. Field of the Invention
The present invention relates to a capacitor. More specifically, the present invention relates to a space-saving capacitor circuit for use in the pixel area of a light emitting display device.
2. Description of the Related Technology
Generally, a flat display panel (FDP), is a display device, in which light emitting elements are arranged between two substrates. The importance of the FDP has been emphasized following the development of multimedia technologies. In response to this trend, various flat displays such as the liquid crystal display (LCD) using a method for scattering light as the liquid crystals are roiled by an applied voltage, the field emission display using a phosphor light emission by electron beam, and the organic light emitting display device (hereinafter, referred to as an OLED device) using light emission of organic materials have been developed.
There are a passive matrix method and an active matrix method to provide control circuits of FDP devices. In the passive matrix method, electrodes are formed crossing each other, and a line is selected in order to drive the device. The active matrix method uses thin film transistors (TFT) for driving light emitting elements of FDP devices. In the active matrix method, the FDP device operates in response to a voltage maintained by capacitance of a capacitor coupled to a gate of the thin film transistor.
FIG. 1 shows a circuit diagram of an OLED device, for example. As shown in FIG. 1, a pixel circuit of the OLED device include an organic light emitting diode OLED, two transistors M1 and M2, and a capacitor Cst. A power voltage VDD is coupled to a source of the driving transistor M2, and the capacitor Cst is coupled between a gate and a source of the driving transistor M2. The capacitor Cst maintains a gate-source voltage of VGS of the driving transistor M2 for a predetermined period. The switching transistor M1 transmits a data voltage from a data line Dm to the gate of the transistor M2 in response to a selection signal from a current scan line Sn. A cathode of the organic light emitting diode OLED is coupled to a reference voltage of Vss and emits a light corresponding to a current applied through the driving transistor M2.
As shown, in the active matrix method, each pixel circuit includes a thin film transistor and a capacitor, and the pixel circuit operates in response to a voltage maintained by capacitance of the capacitor Cst that is coupled to the gate of the thin film transistor. A display quality of the pixel is better when comparing to the passive matrix method because an image corresponding to a data signal is continuously displayed in a frame, and therefore the active matrix method has become very popular these days.
FIG. 2 shows a sectional view of a configuration of a capacitor used in a display panel. A buffer layer 11 is formed on a substrate 10, and a conductive layer 12 forming an electrode of the capacitor is formed on the buffer layer 11. An insulation layer 13 is formed on the conductive layer 12, and a conductive layer 14 forming another electrode of the capacitor is formed on the insulation layer 13. Therefore the capacitor including the conductive layer 12 and the conductive layer 14 is formed. The capacitor is formed in an area forming a pixel circuit, and the horizontal widths of the capacitor are designed in view of the capacitance required according to characteristics of the display panel.
A thin film transistor is formed on a pixel area of the display panel in the active matrix method as well as the capacitor having a predetermined width, which may limit an area for forming display elements. Accordingly, an aperture ratio of the display panel may be decreased in an active matrix display device.
Further, OLED devices tend to use a pixel circuit having more than two capacitors and a plurality of thin film transistors in order to compensate a threshold voltage of a driving transistor. In such devices, the aperture ratio may be further decreased. In response to the trend, there is a high demand for a space-saving capacitor having proper capacitance in the pixel area.
The information disclosed in this Background of the Invention section is only for understanding of the background of the invention, and therefore, unless explicitly described to the contrary, it should not be taken as an admission of prior art.